Indoor radon is a major source of radiation exposure to the world's population and may be an important cause of lung cancer. A recently completed case-control study in Missouri measured radon concentrations in current and former homes of 700 female lung cancer cases and 700 controls. In Gansu, China where inhabitants live in underground dwellings having relatively high levels of radon, a case-control study of lung cancer, radon and air pollution has been initiated. A meta-analysis of case-control studies of lung cancer and indoor radon indicates that the risk from indoor radon is not likely to be greater than the risk predicted from undergound miners, in whom radon has been established as a human carcinogen. The most dramatic health effect of the Chernobyl nuclear reactor accident in the former Soviet Union is the unprecedented increase in thyroid cancer among children. In collaboration with the Department of Energy, exposure to radionuclides from the accident is being evaluated in 15,000 children in Belarus and 50,000 children in the Ukraine who are undergoing annual thyroid examination, laboratory tests and interviews. Villagers living along the banks of the Techa River in Russia were exposed to environmental contamination from high-level radioactive waste. Cancer mortality is being evaluated in a cohort of approximately 28,000 people who received large doses of chronic external and internal radiation from waste dumped into the river from the Mayak nuclear plant.